1. Field of Invention
Embodiments of the invention relate generally to uninterruptible power supplies (“UPS”). More specifically, at least one embodiment relates to a system and method for controlling a DC bus voltage of a UPS.
2. Discussion of Related Art
Because electrical systems are dynamic in nature, control systems employed with power conversion circuitry are generally responsive to changes in the electrical systems. Different approaches may be employed to address various types of changes, for example, transient or steady state changes, changes in voltage and/or current, changes in the connected load, combinations of the preceding examples, etc.
Today, power factor correction of power conversion circuitry (i.e., the phase relation between the current and the voltage used by the power conversion circuitry) is a significant design consideration. In practice, power factor can be improved by reducing harmonics generated by the power conversion circuitry during operation. Generally, power conversion circuitry designed for reduced harmonics, however, must employ a voltage control loop that has a relatively slow response time. The slow response time results in a poor transient response including wider deviations from nominal voltages, for example, greater overshoot and undershoot of a DC bus voltage. Thus, circuit designers attempt to provide a system with acceptable power factor control and acceptable dynamic response while meeting the requirements of applicable harmonic standards.
In the case of a single phase UPS, designers generally provide a control scheme (e.g., DC voltage loop control) with relatively slow frequency response (well below the line frequency of the AC input) in order to meet the harmonic standards. That is, a feedback control system is employed in which the DC bus voltage is compared to a setpoint and adjustments made based on an error between the measured DC bus voltage and the setpoint. The result of the design tradeoff included in this approach is that the DC bus voltage has a relatively poor transient response. The poor transient approach can result in large overvoltages and undervoltages on the DC bus as the DC bus voltage varies, for example, in response to changes in the loading of the UPS output.
Other power conversion applications that use a DC link or bus also face the need to control the transient response of the DC bus in a dynamic system, for example, switch mode power supplies, voltage regulators and motor drives to name a few. Some more recent approaches to the above-described problem in single phase power factor control are a “notch filter method” and a “dead zone digital control method.” Neither of these approaches is satisfactory in a UPS application. For example, the notch filter method is most suitable where a substantially fixed line frequency is found. For this reason, the notch filter method is impractical because many UPS applications do not operate in environments that meet the preceding requirement. The “dead zone” approach is not suitable for many UPS applications because it is not suited for a system in which the DC bus supplies power to an inverter which is connected to a non-linear load, such as a computer load.
Some prior motor control systems have employed a feedforward scheme in which average power at the output of the power conversion circuitry is employed in regulating a DC bus voltage. These approaches, however, are employed in three phase motor control in which the instantaneous load power sums to zero because the motor provides a polyphase, balanced and linear load.